We are studying the electrophysiological properties of leech neuronal sets that are defined by a panel of monoclonal antibodies (mAbs). Different mAbs define sets of different size and all of these sets intersect. A small set may fall entirely within a large set; other sets only overlap partially. 1. In adult ganglia, we identify individual neurons within a mAb-stained set with doulbe-labeling experiments that combine electrophysiological identification with Lucifer yellow injection and antibody staining. Some sets contain motor neurons, sensory neurons and interneurons. 2. We determine the degree of synaptic connectivity between neurons within the same set or between different sets with pairwise intracellular microelectrode recordings. The extent to which different sets overlap is measured in a double-labeling experiment using two different mAbs. Those neurons that localize in several sets and thus carry an assortment of different combinatorial antigens are studied most exhaustively with respect to function and synaptic connectivity. 3. In developmental and tissue culture systems, we will test our mAbs for physiological effects as well as correlate antigen expression with synapse and network formation. Unlike neurotransmitters, the combinatorial markers discussed here are expressed to a significant extent postembryonically. 4. Additional set-specific mAbs will be generated. Do these mAb-defined intersecting sets of neurons correspond to subnetworks subserving different coherent behaviors? Are they the morphological correlate of combinatorial factors hypothesized to mediate synaptic recognition? The simple leech nervous system with its small number of identifiable neurons and identifiable synaptic connections offers an excellent system to correlate this molecular organization with its underlying function.